 //测试计算器功能
 var cal = require ('./index');
 console.log(cal.add(1, 2));        // => 3
 console.log(cal.subtraot(1, 2));     // => -1
 console.log(cal.multiply(1, 2));       // => 2
 console.log(cal.divide(1, 2)) ;     // => 0.5